Is this common or even significant?

I am in my 40's. For years (20 or so) I have had a fairly constant and very subtle "jukebox" of sorts in my cognitive background. That is, I will catch myself humming a tune or even singing part of one while I'm performing my daily activities. I even wake up with a tune in my head. The stranger thing is that the tunes are random (no particular genre) or obscure and usually from the 60's and 70's. I don't need to hear the tune first as a trigger. I just catch myself tapping the beat to, or singing "take the money and run" or "1, 2 ahhh freak out!" etc. I'm not bothered or worried by it. Simply curious if it is common and why I might be stuck in that era.